Medical electrode

ABSTRACT

The present disclosure provides electrodes that possess hydrogels for contacting skin. In embodiments, an electrode of the present disclosure may include a substrate and a conductive composition on at least a portion of a surface of the substrate, the conductive composition including at least one hydrogel and at least one component capable of providing either a cooling or warming sensation upon electrical stimulation. Methods for forming these hydrogels and electrodes are also provided.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/855,292, filed Aug. 12, 2010, now U.S. Pat. No. 8,548,557, the entiredisclosure of which is hereby incorporated by reference herein.

TECHNICAL FIELD

The present disclosure relates to hydrogels suitable for use asconductive compositions, methods of making these compositions, and theuse of these compositions with medical electrodes.

BACKGROUND OF RELATED ART

Hydrogels constitute a broad class of materials which may be completelywater soluble or swell extensively in water, but are not completelywater soluble. They have been used in a variety of biomedicalapplications and may be applied in bulk forms which vary from clear toopaque, and from a relatively stiff to a relatively soft consistency.Sometimes the bulk forms are reinforced by reinforcement members whichmay be woven or non-woven fabrics to increase the composite strengthand/or dimensional stability. Hydrogels have also been used as coatingsfor various biomedical applications.

Medical electrodes are used to transmit electrical signals or currentsbetween the body of a patient and external medical equipment. Theseelectrodes may include a conductive composition adhered to or otherwisein contact with, the skin of the patient, and a conductor, which iselectrically connected to the conductive composition and to the externalmedical equipment.

Hydrogels for use as conductive compositions with medical electrodesremain desirable.

SUMMARY

The present disclosure provides electrodes that possess componentscapable of enhancing patient comfort upon application of an electrode totissue, including skin.

In embodiments, an electrode of the present disclosure may include asubstrate and a conductive composition on at least a portion of asurface of the substrate, the conductive composition including at leastone hydrogel and at least one component capable of providing either acooling or warming sensation upon contact with tissue of a patient.

Where the hydrogel includes a warming component, the warming componentmay be present in an amount of from about 0.01% by weight to about 1% byweight of the hydrogel.

Where the hydrogel includes a cooling component, the cooling componentmay be present in an amount of from about 0.01% by weight to about 20%by weight of the hydrogel.

Hydrogels of the present disclosure may include electrolytes and otheradditives. Where the hydrogel includes an electrolyte, the electrolytemay be present in an amount of from about 0.1% by weight to about 15% byweight of the hydrogel.

Methods for producing electrodes and the components thereof are alsoprovided, as are methods for their use.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top plan view of a medical electrode including theconductive composition of the present disclosure.

FIG. 2 is a cross-sectional view of the medical electrode of FIG. 1.

FIG. 3 is a cross-sectional view of a snap medical electrode.

FIG. 4 is a cross-sectional view of an alternate medical electrode ofthe present disclosure.

DETAILED DESCRIPTION

Any adhesive application, including those involving tissue, are withinthe purview of the hydrogel compositions of the present disclosure. Inembodiments, hydrogels may be utilized as adhesives and/or conductivecompositions for medical electrodes. The hydrogels of the presentdisclosure may include components that provide either a cooling orwarming sensation upon contact with tissue of a patient, which may bedesirable for a patient.

As used herein, the term “hydrogel” may refer to a wide variety ofpolymer-based compositions. These materials may be synthesized forexample from monomer(s) or from monomer(s) mixed with polymer(s) orcross-linked polymer solutions in water. They may be obtained bychemical modification of existing polymer(s) or by adding water toexisting dry polymers.

Any biocompatible hydrogel may be utilized in accordance with thepresent disclosure. Generally speaking, a hydrogel according to thepresent disclosure may include a coherent, three-dimensional aqueouspolymer system capable of imbibing water without liquefying. Inembodiments, insolubility in water may be provided by crosslinking thehydrogel polymer. In embodiments, hydrogels or water-containing gels ofthe present disclosure may include water and various chemical substancesincluding gelatin; polysaccharides; crosslinked acrylamide polymers,hydroxyethylmethacrylate polymers; crosslinked polyhydroxyethylacrylate;polymerized, crosslinked 2-acrylamido-2-methylpropane sulfonic acidpolymers or one of their salts such as the sodium or potassium type;crosslinked polyvinylpyrrolidone; polyacrylic acid; copolymers of theaforementioned monomers with each other, copolymers of theaforementioned monomers with other polymers such as polystyrene or othernon-hydrogel-forming polymers, one or more salts of the foregoing, andcombinations thereof.

For example, by cross-linking homopolymers of an acrylamide derivativesuch as 2-acrylamido-2-methylpropanesulfonic acid or one of its salts,hydrogels may be formed. Copolymers thereof may also be formed in thesame way with acrylamide. Cross-linked homopolymers of acrylic acid andof methacrylic acid, their salts and copolymers may also be formed, asmay other acrylic cross-linked homopolymers and copolymers.

Hydrogels of the present disclosure derive their adhesive properties inpart from their ability to absorb water. When a relatively dry body ofhydrogel contacts moisture, such as the moisture in tissue, particularlyinternal tissue, or any other moist surface, it develops an aggressiveadhesive nature. When the polymer of the hydrogel is crosslinked to anadequate degree, the bulk hydrogel is sufficiently strong, even whenswelled with additional liquid, to provide adhesive support for pacingleads, thereby establishing extended connection of the lead to tissue.However, excessive crosslinking decreases the tack of the hydrogel. Toolittle crosslinking decreases its cohesive strength. Thus, inembodiments, a crosslinking agent may be utilized in forming the polymersuitable as a hydrogel of the present disclosure.

In some embodiments, a suitable hydrogel for use as a conductivecomposition may include a copolymer. Non-limiting examples of suitablecopolymers may include a first monomer, such as a mixture of acrylicacid and a salt thereof, and a second monomer, such as one of moremonomers selected from CH₂═CHC(O)XR, in which X is O or NH and R is anunsubstituted or substituted alkyl group of 1 to 5 carbon atoms. Thehydrogel may also include water; an electrolyte or mixture ofelectrolytes; a polymerization initiator; a neutralizer a such as sodiumhydroxide; optionally, a crosslinking agent; and optionally, athickener.

In embodiments, a first monomer which may be used to form a copolymerfor use in a hydrogel includes acrylic acid, a salt thereof, or amixture thereof. The copolymer thus produced by polymerization includesacid acrylate moieties (—CO₂H and/or —CO₂M, in which M is a cation suchas sodium ion, potassium ion, lithium ion, ammonium or substitutedammonium ion, etc.) directly attached to the polymer backbone.

In embodiments, a copolymer utilized in a hydrogel of the presentdisclosure may include a second monomer which may be one of moremonomers of the formula CH₂═CHC(O)XR, in which X is O or NH and R is anunsubstituted or substituted alkyl group of 1 to 5 carbon atoms. Thepolymer produced by this polymerization includes groups of the structure—C(O)XR directly attached to the polymer backbone.

Suitable unsubstituted alkyl groups are methyl, ethyl, n-propyl,n-butyl, and n-pentyl. Suitable substituents that may be present in asubstituted alkyl group are halo (such as F, Cl, or Br) cyano,carboxylic acid and salts thereof (i.e., —CO₂H or —CO₂M, in which M is acation), phosphate and salts thereof, and sulfonic acid and saltsthereof. An example of such a substituted alkyl group is(3-sulfopropyl)acrylic acid ester, potassium salt. Suitable secondmonomers include 2-acrylamido-2-methylpropane sulfonic acid(CH₂═CH—CONHC(CH₃)₂—CH₂—SO₃H) and/or a salt thereof. Suitable saltsinclude the sodium, lithium, potassium, ammonium, and substitutedammonium salts, and mixtures thereof.

In embodiments, the second monomer utilized in a copolymer component ofa hydrogel of the present disclosure is 2-acrylamido-2-methylpropanesulfonic acid sodium salt (NaAMPS) (CH₂═CH—CONHC(CH₃)₂—CH₂—SO₃ ⁻M⁺).Thus, in some embodiments, the first monomer utilized in a copolymercomponent of a hydrogel of the present disclosure may include a mixtureof acrylic acid and sodium acrylate, and the second monomer may includesodium 2-acrylamido-2-methylpropane sulfonate.

The first monomer (acrylic acid and/or salt or salt thereof, calculatedas acrylic acid) may be present in an amount of from about 8 weight % toabout 85 weight % of copolymer in the hydrogel, in embodiments fromabout 10 weight % to about 80 weight %, of the total amount of thecopolymer in the hydrogel. The second monomer, in embodiments NaAMPS,may be present in an amount of from about 15 weight % to about 92 weight% of the copolymer in the hydrogel, in embodiments from about 20 weight% to about 90 weight % of the copolymer in the hydrogel.

Optionally, an effective amount of a cross-linking agent or mixture ofcross-linking agents may be utilized to form the copolymer component ofa hydrogel of the present disclosure. An effective amount ofcross-linking agent is an amount that produces a conductive compositionwith the desired physical properties, such as coherence and adhesion,and electrical properties. Although the amount required will depend on,for example, the molecular weight of the cross-linking agent, the numberof ethylenically unsaturated, free radical polymerizable groups presentin the cross-linking agent, the amount of free radical polymerizablemonomers present in the monomer mix, when the cross-linking agent ispresent, the amount of crosslinking agent will be present in an amountof from about 0.01 weight % to 1 weight % of the copolymer utilized inthe hydrogel, in embodiments from about 0.02 weight % to 0.08 weight %of the copolymer utilized in the hydrogel.

Suitable cross-linking agents include free radical polymerizablemonomers that possess more than one ethylenically unsaturated, freeradical polymerizable group. Numerous crosslinking agents polymerizableby free-radical initiated polymerization are within the purview of thoseskilled in the art. Crosslinking agents include, for example,bis-acrylamides and methacrylamides, such as N,N′-methylenebis-acrylamide; acrylate and methacrylate esters of polyols, such as,ethylene glycol diacrylate and dimethacrylate, diethylene glycoldiacrylate and dimethacrylate, trimethylolpropane triacrylate andtrimethacrylate, ethoxylated trimethylolpropane triacrylate andtrimethacrylate; pentaerythritol triacrylate and trimethacrylate,pentaerythritol tetraacrylate and tetramethacrylate, and polyethyleneglycol diacrylates and dimethacrylates, such as the diacrylates anddimethacrylates of polyethylene glycols having a molecular weight offrom about 200 to about 600. In embodiments, a suitable crosslinkingagent may include N,N′-methylene bis-acrylamide [CH₂═CHCONH)₂CH₂].

In embodiments, a polymerization initiator may be utilized with thefirst monomer and second monomer to form a copolymer for use in ahydrogel of the present disclosure. An effective amount of apolymerization initiator may be combined with the monomers to form sucha copolymer. As used herein, an effective amount is an amount thatproduces efficient polymerization of the monomers under polymerizationconditions to produce a hydrogel suitable for use as a conductivecomposition. Numerous free radical polymerization initiators are withinthe purview of those skilled in the art. The polymerization initiatormay be a single compound or a mixture of compounds. Thermal and/or photofree radical polymerization initiators, for example, may be used.

Suitable thermal free radical polymerization initiators include azocompounds, such as 2,2-azobisisobutyronitrile (AIBN). Suitable photofree radical polymerization initiators are disclosed in “Photoinitiatorsfor Free-Radical-Initiated Photoimaging Systems,” by B. M. Monroe and G.C. Weed, Chem. Rev., 93, 435-448 (1993) and in “Free RadicalPolymerization” by K. K. Dietliker, in Chemistry and Technology of UVand EB Formulation for Coatings, Inks, and Paints, P. K. T. Oldring,ed., SITA Technology Ltd., London, 1991, Vol. 3, pp. 59-525. Suitablefree radical photo polymerization initiators include, for example,1-hydroxycyclohexylphenyl ketone (HCPK, IRGACURE® 184);2-hydroxy-2-methyl-1-phenylpropan-1-one (DAROCUR® 1173);2-hydroxy-1-[4-(2-hydroxyethoxy)phenyl]-2-methyl-1-propan-1-one(IRGACURE®2959), 2,2-dimethoxy-2-phenylacetophenone (benzildimethylketal, BDK, IRGACURE®651), benzophenone, a mixture of 50 weight %benzophenone and 50 weight % of 1-hydroxycyclohexylphenyl ketone(IRGACURE® 500), and combinations thereof.

The polymerization initiator may be present in a copolymer utilized in ahydrogel in an amount less than about 1 weight % of the copolymer, inembodiments less than about 0.7 weight % of the copolymer, in otherembodiments less than about 0.4 weight % of the copolymer.

The hydrogel utilized as a conductive composition may also include aneutralizer. Bases such as hydroxides, amines, Lewis bases, and mixturesthereof may be used as neutralizers. Non-limiting examples ofneutralizers include ammonium hydroxide, sodium hydroxide, potassiumhydroxide, lithium hydroxide, combinations thereof, and the like. If theacrylic acid and/or the second monomer, such as the2-acrylamido-2-methylpropane sulfonic acid, are included as monomers informing a copolymer for use in the hydrogel, it may be desirable to addneutralizer to neutralize some of the acid so that the pH of the mixtureis from about 3 to about 6.5.

Where utilized, a neutralizer may be present in an amount from about 2weight % to about 8 weight % of the hydrogel.

In addition to a free radical initiator, small amounts of free radicalpolymerization inhibitors may be present with one or more of themonomers, and/or the crosslinking agent, and/or may be added to themixture to prevent premature polymerization of the reaction mixture.Suitable free radical polymerization inhibitors include, for example,hydroquinone, 4-methoxyphenol, di-t-butyl-p-cresol, pyrogallol, t-butylcatechol, benzoquinone, 4,4′-thio-bis-(3-methyl-6-t-butylphenol), and2,2′-methylene-bis-(4-methyl-6-t-butylphenol). When present, the amountof the polymerization inhibitor may be from about 0.01 weight % to about5 weight % of the hydrogel.

In some embodiments a thickener may be added to the hydrogel. Suitablethickeners include rheological modifiers which permit tailoring theviscosity of the hydrogel to permit its use as a conductive compositionwith a medical electrode. Non-limiting examples of such thickenersinclude silica, gums including xanthan gum, polymers including polyvinylpyrrolidone (PVP), polyacrylamides, polyacrylic acid (including thosesold under the name CARBOPOL®), salts thereof, combinations thereof, andthe like. Where utilized, a thickener may be present in a hydrogel ofthe present disclosure in an amount from about 0.1 weight % to about 8weight % of the hydrogel, in embodiments from about 0.5 weight % toabout 5 weight % of the hydrogel.

An example of a suitable polymer which may be utilized as the hydrogelincludes RG-63B, commercially available from Covidien.

In use, such a hydrogel of the present disclosure may contain thepolymer or copolymer, and any other additives, including componentsutilized to form the copolymer, in an amount from about 20% by weight toabout 97% by weight of the hydrogel, with the balance being water and/ora humectant.

As noted above, the above gels may be formed by free radicalpolymerization. Free radical polymerization may be initiated by, forexample, heating the mixture when a thermal free radical polymerizationinitiator is present in the mixture, or exposing the mixture to actinicradiation when a photoinitiated free radical polymerization initiator ispresent in the mixture. Any convenient source or sources of actinicradiation providing wavelengths in the region of the spectrum thatoverlap the absorption bands of the photoinitiated free radicalpolymerization initiator can be used to activate polymerization. Theradiation can be natural or artificial, monochromatic or polychromatic,incoherent or coherent, and for high efficiency should correspondclosely in wavelengths to the absorption bands of the polymerizationinitiator. Conventional light sources include fluorescent lamps, mercuryvapor lamps, metal additive lamps, and arc lamps. Useful lasers arethose whose emissions fall within or overlap the absorption bands of thephotoinitiated free radical polymerization initiator. Although, ifdesired, the mixture may be degassed before polymerization and/or thepolymerization may be carried out under an inert atmosphere, it is notnecessary to degas the mixture before polymerization or to carry out thepolymerization under an inert atmosphere.

Other hydrogels which may be utilized in accordance with the presentdisclosure include those disclosed in U.S. Pat. Nos. 5,354,790,5,143,071, 4,657,009 and 4,657,023, the disclosures of each of which areincorporated herein by reference in their entireties for all purposes.For example, hydrogels for use in accordance with the present disclosuremay be based upon polyethylene oxide (PEO), polyvinyl pyrrolidone (PVP),natural polysaccharides such as Karaya gum, synthetic polysaccharides,combinations thereof, and the like.

For example, in embodiments, the hydrogel may be formed from ahomogeneous aqueous mixture including water and a crosslinked polymersuch as polyethylene oxide (PEO). In other embodiments, a hydrogel maybe formed from a cohesive uniform mixture of water, an electrolyte, acrosslinked poly(vinyl pyrrolidone) (PVP), and a viscosity-enhancingpolymer. It should be noted that these hydrogel compositions may existas a multiphase system including high molecular weight macromoleculeswhich are present in the uniform mixture. An irradiative crosslinkingprocess may be utilized to form these hydrogels; the irradiation“freezes” in place these microphase regions to provide a stablematerial.

As noted above, these PEO and/or PVP based hydrogels may be produced byexposing an aqueous mixture of at least one water-soluble high molecularweight linear polymer, e.g., PEO or PVP, to a dose of high energyionizing radiation, which is effective to form a gel-like material. Forexample, a homogeneous aqueous mixture of PEO, present at aconcentration of from about 7 to about 35 weight %, in embodiments fromabout 10 to about 25 weight %, may be used. Alternatively, an aqueousmixture of PVP and a viscosity-enhancing hydrophilic polymer may beemployed. It has been found that irradiation of these polymeric mixturesproduce gels which are cohesive, adhesive, sufficiently tacky, and yetnon-stringy.

For a conductive formulation, the homogeneous aqueous mixtures describedabove may be prepared with an effective amount of a water-solubleelectrolyte. Irradiation of these electrolyte-containing mixtures mayproduce conductive sheets having the desired non-stringy surfaceproperties.

PVPs which may be used for forming a hydrogel in accordance with thepresent disclosure may include a polymer of N-vinyl-2-pyrrolidone havinga weight average molecular weight (M_(W)) of from about 200 kD to about2,000 kD, in embodiments from about 500 kD to about 1,500 kD, inembodiments from about 750 kD to about 1,250 kD.

Homogeneous aqueous mixtures having from about 7 to about 35 weightpercent of PVP, in embodiments from about 10 to about 25 weight percentof PVP, may be suitable for forming hydrogels.

Irradiation crosslinking of PVP mixtures may occur utilizing meanswithin the purview of those skilled in the art, including thosedisclosed in U.S. Pat. No. 4,699,146, the entire disclosure of which isincorporated herein by reference in its entirety for all purposes.

In embodiments, a hydrophilic polymer, having a weight average molecularweight in excess of about 100 kilodaltons, may be added to the mixturein amounts from about 0.5 weight percent to about 4 weight percent ofthe mixture. This hydrophilic polymer can enhance the viscosity of theconductive polymeric mixture such that the final viscosity is restoredto a degree providing coatable, extrudable viscous aqueous polymericmixtures. Furthermore, when exposed to radiant energy at an effectivedose, a film of the extrudable aqueous mixture is transformed into acohesive gel-like solid having the desirable highly conductive adhesiveproperties of the hydrogels.

As stated above, the viscosity-enhancing hydrophilic polymer should havea sufficiently high weight average molecular weight, M_(W). In general,such a polymer should have a M_(W) in excess of about 100 kD, inembodiments as high as 15 million daltons. Such a polymer may be derivedfrom natural, synthetic, or semisynthetic sources; it may be linear,branched, crosslinked, noncrosslinked, water-soluble, or waterinsoluble, so long as it is hydrophilic in nature.

Examples of suitable synthetic materials useful as viscosity-enhancersinclude, but are not limited to, polyacrylamide, poly(vinyl alcohol), apolyacrylate salt, poly(ethylene oxide), poly(ethylene imine),polyacrylamide sulfonic acid or their salts, polyacrylonitrile,hydrophilic derivatives, mixtures, blends, or copolymers thereof. Inembodiments, the hydrophilic polymer utilized as a viscosity enhancermay be a poly(ethylene oxide) having a M_(W) of from about 500 to about10,000 kD, in embodiments about 900 kD.

In embodiments, a suitable semisynthetic polymer which may be used as aviscosity-enhancer includes a derivative of cellulose. For example,methylcellulose, ethylcellulose, carboxymethylcellulose,hydroxyethylcellulose, combinations thereof, and the like, may be used.In some cases, a carboxymethylcellulose (CMC) having a weight averagemolecular weight of about 700,000 daltons, may be used.

Other suitable macromolecules which may be used as a viscosity-enhancerinclude those derived from natural sources, such as starch, agar,dextran, dextrin, carrageenan, xanthan, guar, their derivatives, orcombinations thereof.

The above viscosity-enhancers may be added to an aqueous electrolyte-PVPmixture in amounts of from about 1 weight % to about 2 weight % of themixture. Their inclusion may increase the viscosity of the mixture to atleast about 8,000 centipoise (cPs), in embodiments from about 10,000 cPsto about 20,000 cPs.

The poly(vinyl pyrrolidone) formulations useful in these applicationsinclude those incorporating and binding high concentrations of waterwhile maintaining adequate surface tack (adhesiveness), sufficientstrength (cohesiveness), and substantial non-stringiness. Thepolymer/salt/viscosity-enhancer/water mixture should be viscous enoughto be extrudable and to form into a sheet-like configuration, e.g., aliquid film of about 0.1 to about 2 mm thickness, before crosslinking.In embodiments, the viscosity of the aqueous mixtures exceed about 8,000cps.

After the viscous mixture is applied or cast to the desired thickness,it is then subjected to crosslinking high energy irradiation, such as ahigh energy electron flux as produced by an electron accelerator or VanDe Graaf generator, to form a hydrogel. In general, alpha particles,beta particles, gamma rays, X-rays, electron beams, or high energyultraviolet radiation may be used to initiate or precipitate thecrosslinking of polymer chains. The major requirement is that the beamof electrons be of sufficient energy to completely penetrate themixture, so that the mixture receives a radiation dose effective tocrosslink the entire cross section of the sample. Properdose/energy/thickness relationships are within the purview of thoseskilled in the art of radiation processing. To achieve the desireddegree of uniform crosslinking, doses of from about 0.5 Mrads to about4.5 Mrads, in embodiments from about 0.6 Mrads to about 2 Mrads, inembodiments from about 0.7 Mrads to about 1.8 Mrads may be utilized,depending upon the selected polymer, its concentration,viscosity-enhancer and its concentration, the selected salt and itsconcentration, and the presence or absence of any functional,therapeutic agents, or other additives.

The formation of a polyethylene oxide formulation is similar to thosedescribed above for a PVP formulation. Suitable PEO polymers for use informing a hydrogel in accordance with the present disclosure may havemolecular weights from about 200 kilodaltons (kD) to about 5,000 kD, inembodiments from about 750 kD to about 3,000 kD, in other embodimentsfrom about 1,000 kD to about 2,500 kD.

Useful PEOs include those incorporating and binding high concentrationsof water while maintaining adequate surface tack (adhesiveness), toavoid leaving a residue. The starting water soluble linear polyethyleneoxide should have a molecular weight high enough to form a viscoussolution for processing and readily crosslink. Generally, polymers withweight average molecular weights (M_(W)) of from about 0.2×10⁶ to about10×10⁶ Daltons, in embodiments from about 0.5×10⁶ to about 5×10⁶Daltons, may be used. The concentration of polymer may be from about 3to about 35 weight %, in embodiments from about 4 to about 20 weight %,of the overall mixture, depending upon its molecular weight. Inembodiments, the concentration of polymer may be less than about 35%, sothat the polymer may not be overly brittle. The polymer water solutionshould be viscous enough to form into a sheet-like configuration, e.g.,a liquid film of about 0.2 to 4 mm thickness, before crosslinking.Illustrative viscosities are from about 2,000 to 2,000,000 cPs.

A continuous sheet of hydrophilic gel of from about 10 to about 150 mils(0.254-381 mm) may be formed. After the viscous solution is applied orcast to the desired thickness, it is then subjected to crosslinking highenergy irradiation, such as a high energy electron flux as produced byan electron accelerator. If conditions are selected which excludeatmospheric oxygen, gamma radiation may be used. The major requirementis that the beam of electrons be of sufficient energy to completelypenetrate the solution, so that the solution receives a radiation doseeffective to crosslink the entire cross section of the sample. Properdose/energy/thickness relationships are within the purview of thoseskilled in the art. To achieve the desired degree of uniformcrosslinking, i.e., effective to convert the viscous polymer solutioninto a viscoelastic solid gel, doses of from about 0.20 to about 5.0Mrads, in embodiments from about 0.75 to about 2.0 Mrads, may be used,depending upon the selected polymer molecular weight, and itsconcentration, any functional or therapeutic additives included in theviscous polymer solution, and the like. Generally, higher polymerconcentrations require high irradiation doses to produce an acceptableviscoelastic solid gel.

In other embodiments, the hydrogel may be formed from naturallyoccurring materials such as gum karaya, gum acacia, locust bean gum andother polysaccharides, and synthetically formulated polysaccharides suchas guar and celluloses including cellulose derivatives such ascarboxymethyl cellulose, as well as combinations thereof. The hydrogelmay also be formed from synthetic polymers such as polyacrylamide andits congeners, as well as polyacrylic acids. Such synthetic polymers mayhave weight average molecular weights (M_(W)) of from about 250,000 toabout 4,000,000, in embodiments from about 450,000 to about 1,000,000.When monomers such as acrylic acid or acrylamide are polymerized, it maybe desirable to use activators in the formation of the hydrogels.Activators, which may be used during polymerization include ferroussulfate, sodium metabisulfite, potassium persulfate, combinationsthereof, and the like. Suitable amounts of activators are within thepurview of those skilled in the art.

The synthetic polymers and/or synthetic or natural gums and otherpolysaccharides may form a solid phase of the matrix forming thehydrogel. The liquid phase of the matrix forming the hydrogel mayinclude hydric alcohols such as glycerol and/or propylene glycol, and/orwater, most commonly water. Solutions or emulsions of saccharides and/orpolysaccharides and/or proteins may be used to plasticize the matrixforming the hydrogel. Alternatively, a combination of a solution oremulsion of polysaccharides, saccharides or proteins may be used in theliquid phase of the matrix forming the hydrogel.

The hydrogel thus includes a gel phase including a synthetic polymermixture, a large molecular weight polysaccharide matrix, and/or a matrixincluding a large molecular weight polysaccharide in combination with asynthetic polymer. The hydrogel may have a solids content of from about2% to about 50% by weight of the hydrogel, in embodiments from about 10%to about 40% by weight of the hydrogel. The liquid phase of thehydrogel, including water, may be present in an amount of from about 50%to about 98% by weight of the hydrogel, in embodiments from about 60% toabout 90% by weight of the hydrogel.

In embodiments, a vinyl acetate dioctyl maleate copolymer may be used informing the hydrogel. In other embodiments, suitable gum materials whichmay be used in forming the hydrogel include starch graft copolymersderived from corn starch and acrylonitrile, including graft terpolymersof starch, acrylamide and sodium acrylate known asstarch-g-poly(acrylamide-co-sodium acrylate). Thestarch-g-poly(acrylamide-co-sodium acrylate) material may be used aloneto form the substrate or it may be used in combination with a syntheticgum such as acrylamide or a natural gum such as karaya.

The monomers and any additional components described above for forming ahydrogel may be mixed and spread or coated as a layer on a releaseliner, for example a siliconized release substrate such as siliconecoated polyethylene terephthalate film, or other substrate prior topolymerization. In other embodiments, the hydrogel may be formed andthen applied to a release liner and/or substrate for use with anelectrode.

As noted above, a variety of electrolytic substances may be added to themixtures in amounts sufficient to produce conductive products. Suitableelectrolytes include ionizable inorganic salts, organic compounds, orcombinations of both. Examples of such salts include, but are notlimited to, lithium chloride, magnesium chloride, sodium chloride,potassium chloride, magnesium acetate, ammonium acetate,monoethanolamine acetate, diethanolamine acetate, sodium lactate, sodiumcitrate, ammonium sulfate, magnesium sulfate, calcium sulfate,combinations thereof, and the like. The electrolyte should be stable andinert upon dissolving in any aqueous mixture utilized to form ahydrogel, and any subsequent radiation crosslinking step for thosehydrogels formed thereby. Although virtually any amount of electrolytemay be present in the hydrogel, it may be desirable to have theelectrolyte present at a concentration of from about 0.1 to about 15weight % of the hydrogel, in embodiments from about 0.7 to about 10weight % of the hydrogel, in embodiments from about 1 weight % to about8 weight % of the hydrogel.

Following polymerization of the components utilized to form a hydrogel,the resulting conductive composition may transferred to a conductivesubstrate. Alternatively, the conductive composition may be adhered to aconductive substrate, and the release liner left in place to protect theconductive composition until it is ready for use.

Sensation Enhancing Component

In accordance with the present disclosure, a hydrogel utilized with anelectrode of the present disclosure also possesses one or morecomponents which may be utilized to provide either a cooling or warmingsensation upon placement on a patient's skin, optionally upon theintroduction of electrical stimulation. Such a component may be referredto, in embodiments, as a sensation enhancing component. The sensationenhancing component may be added to the monomers utilized to form thehydrogel, thereby becoming incorporated within the gel portion of thehydrogel matrix upon polymerization. A sensation enhancing component mayalso be imbibed in the hydrogel after formation of the gel.Alternatively, a sensation enhancing component may be added to a polymermix prior to crosslinking with ionizing radiation or prior to thermosetgel formation.

As noted above, in embodiments, the sensation enhancing component mayprovide a cooling effect and/or sensation upon use and may be referredto, in embodiments, as a cooling component. Examples of suitable coolingcomponents include, but are not limited to, menthol, camphor,eucalyptol, icilin, methyl lactate, N-ethyl-p-menthane-3-carboxamide,combinations thereof, and the like. Where utilized, the coolingcomponent may be present in a hydrogel of the present disclosureutilized with an electrode in an amount of from about 0.01% by weight toabout 20% by weight of the hydrogel, in embodiments from about 0.5% byweight to about 10% by weight of the hydrogel, in embodiments from about1% by weight to about 5% by weight of the hydrogel. In embodiments,menthol may be present in a hydrogel of the present disclosure utilizedwith an electrode in an amount of about 20% or less to reduce skinirritation.

In other embodiments, the sensation enhancing component may provide awarming effect and/or sensation upon use and may be referred to, inembodiments, as a warming component. Examples of suitable warmingcomponents include, but are not limited to, capsaicin, nonivamide,cinnamaldehyde, combinations thereof, and the like. Where utilized, thewarming component may be present in a hydrogel of the present disclosureutilized with an electrode in an amount of from about 0.01% by weight toabout 1% by weight of the hydrogel, in embodiments from about 0.1% byweight to about 1% by weight of the hydrogel, in embodiments from about0.5% by weight to about 1% by weight of the hydrogel.

The conductive composition, including at least one hydrogel and at leastone component capable of providing either a cooling or warming sensationupon electrical stimulation, may be pleasing to a patient to which anelectrode of the present disclosure has been applied. A patientexperiencing a cooling or warming sensation may also communicate thissensation to a health care provider, thereby providing feedback to thehealth care provider that the electrode is, indeed, correctly appliedand functioning as intended.

Other Additives

Other conventional ingredients of conductive compositions may be presentin the hydrogel. For example, humectants and medicinal agents, includingantimicrobials, antiseptics, analgesics, disinfectants, and the like,may be added to a hydrogel. Suitable additives include, but are notlimited to, aloe, centella asiatica, echinacea, ginkgo biloba, ginseng,hyssop, combinations thereof, and the like. Such additional additivesmay be present in a hydrogel of the present disclosure utilized with anelectrode in an amount of from about 0.1% by weight to about 10% byweight of the hydrogel, in embodiments from about 0.5% by weight toabout 5% by weight of the hydrogel, in embodiments from about 1% byweight to about 4% by weight of the hydrogel. In embodiments, suchadditional additives may be present in a hydrogel of the presentdisclosure utilized with an electrode in an amount of about 3%.

Water is present in the mixture. The amount of water includes any waterpresent in any of the ingredients and any water added with ingredientsthat are in water solution, such as the monomers, the crosslinkingagent, the neutralizer, etc. In embodiments, humectants may be added tothe water phase of a hydrogel utilized as a conductive composition in anelectrode of the present disclosure. Humectants which may be usedinclude non-volatile, non-toxic, water soluble or water miscible viscousliquids at room temperature. Suitable humectants include, but are notlimited to, polyhydric alcohols such as glycerol, sorbitol, ethyleneglycol, propylene glycol, polyethylene glycols (PEG) of varyingmolecular weights including PEG 300, PEG 400 and PEG 600, polypropyleneglycols, combinations thereof, and the like. The humectant may beutilized in combination with water or without water. Where utilized withwater, the ratio of water to humectant may be from about 1:10 to about10:1.

As noted above, in some embodiments, a hydrogel of the presentdisclosure may contain the polymer or copolymer and any other additivesdescribed herein in an amount from about 20% by weight to about 97% byweight, with the balance being water and/or a humectant in an amountfrom about 3% to about 80% by weight of the hydrogel.

Exemplary Formulations

The Tables below provide exemplary hydrogels of the present disclosure,which include the above warming or cooling components described above.These formulations are for illustration only; the present disclosure isnot limited thereto.

TABLE 1 Hydrogel with 5% Menthol (E-Beam Cured) Compound Weight % Water90.84 Polyox 301 3.70 Methyl paraben 0.20 Propyl paraben 0.05 PEI CorcatP-600 0.21 Menthol 5.00 100.00 Polyox 301 = an ethylene oxide having amolecular weight of about 400,000, commercially available from UnionCarbide PEI Corcat P-600 = a polyethylimine having a molecular weight offrom about 40,000 to about 60,000, commercially available from CordovaChemcial.

TABLE 2 Hydrogel with Menthol, Camphor, Aloe (E-Beam Cured) CompoundWeight % Water 89.74 Polyox 301 3.70 Methyl paraben 0.20 Propyl paraben0.05 PEI Corcat P-600 0.21 Menthol 3.00 Camphor 3.00 Aloe 0.10 100.00Polyox 301 = is an ethylene oxide having a molecular weight of about400,000 commercially available from Union Carbide PEI Corcat P-600 = apolyethylimine having a molecular weight of from about 40,000 to about60,000, commercially available from Cordova Chemcial.

TABLE 3 Hydrogel with 3% Menthol (UV Cured) Compound Weight % 58% NaAMPSsoln. 46.34 1% MBA in DI water 3.04 Acrylic acid 2.84 Glycerol 41.61 50%NaOH in H₂O soln. 0.12 Silica 2.54 3% Irg.184 in 2-PrO 0.51 Menthol 3.00100.00 1% MBA in DI water = 1% N,N′-methylene bis-acrylamide indeionized water 3% Irg.184 in 2-PrOH = 3% 1-hydroxycyclohexylphenylketone (commercially available as IRGACURE ® 184) in 2-propanol

TABLE 4 Hydrogel with 2% Menthol (UV Cured) Compound Weight % 58% NaAMPSsoln. 46.34 1% MBA in DI water 3.04 Acrylic acid 2.84 Glycerol 42.61 50%NaOH in H₂O soln. 0.12 Silica 2.54 3% Irg.184 in 2-PrOH 0.51 Menthol2.00 100.00 1% MBA in DI water = 1% N,N′-methylene bis-acrylamide indeionized water 3% Irg.184 in 2-PrOH = 3% 1-hydroxycyclohexylphenylketone (commercially available as IRGACURE ® 184) in 2-propanolMedical Electrodes

Medical electrodes transmit electrical signals or currents to or from apatient's skin and an external medical apparatus. Medical electrodes arewithin the purview of those skilled in the art. These electrodes mayinclude a conductive composition such as a hydrogel of the presentdisclosure on a substrate. The layer of conductive composition can beadhered to or contacted with the skin of the patient. The medicalelectrode may also include a conductive interface that is electricallyconnected to the layer of conductive composition and adapted to beelectrically connected to an item of external medical equipment. Formany applications, the conductive composition should be sufficientlyadhesive to adhere to the patient's skin, i.e., be a conductiveadhesive. The configuration of the electrode and the adhesive propertiesrequired will depend on the intended application, such as whether theelectrode is a transmission electrode, i.e., an electrode that sendselectric currents or signals to the patient's body, or a sensing ormonitoring electrode, i.e., an electrode that sends electrical signalsfrom the patient's body to external medical equipment.

FIG. 1 and FIG. 2 show a medical electrode 10 on release liner 12.Release liner 12 is a release paper or film of a waxed or coatedplastic, such as a silicone coated polyethylene terephthalate film,which may be used to protect medical electrode 10 before application ofthe electrode to a skin surface.

Electrode 10 includes a layer of a hydrogel of the present disclosure asconductive composition 14. Electrode 10 also includes conductiveinterface 16 having a conductor member with a conductive portion 18contacting layer of conductive composition 14 and tab portion 20extending beyond layer of conductive composition 14 for mechanical andelectrical contact with external medical equipment, such as aelectrocardiogram monitoring (ECG) machine, an electroencephalogram(EEG) machine, or a transcutaneous electrical nerve stimulation (TENS)machine (not shown). Conductive interface 16 includes conductive layer24, coated on at least side 22 of conductive interface 16. Conductivelayer 24 contacts layer of conductive composition 14. Medical electrode10 can be used either as a diagnostic electrocardiogram (ECG or EKG)electrode or as a transcutaneous electrical nerve stimulation (TENS). Inuse, release liner 12, if present, is removed. The layer of conductivecomposition 14 of electrode 10 is applied to the surface of thepatient's skin and electrically connected to the external medicalequipment.

FIG. 3 shows a cross-section of snap medical electrode 30 on a releaseliner. Electrode 30 has nonconductive backing 32, which has opening 33covered by snap 34 through which eyelet 35 protrudes. Snap 34 is securedto eyelet 35. Together snap 34 and eyelet 35 provide at least part of aconductive interface adapted to provide an electrical connection betweena flexible conductive layer 36 and the external medical equipment (notshown). Eyelet 34 and backing 32 are covered by flexible conductivelayer 36 which, in embodiments, may be made of a material such as carbonvinyl. A hydrogel of the present disclosure may be utilized as aconductive composition 40 and adhered to conductive layer 36.Alternatively, a hydrogel of the present disclosure may be utilized as aconductive composition 40 and adhered to backing 32, thereby omittingconductive layer 36. Release liner 38 protects the conductivecomposition 40 prior to use. In embodiments, a complete or partial layerof silver and/or a silver salt such as silver chloride may be placedbetween conductive composition 40 and conductive layer 36 (not shown).

In addition, in embodiments, conductive layer 36 and non-conductivelayer 32 may possess contiguous windows adjacent each other in eachlayer (not shown) permitting the visualization of conductive composition40 during use so that changes in color, opacity, and the like may beobserved with an electrode in place on a patient.

FIG. 4 shows a cross-section of an alternate medical electrode 100 on arelease liner 112, which may be a polyester film or any other materialsuitable for use as a release liner. Electrode 100 includes a layer of ahydrogel of the present disclosure as conductive composition 114. Inembodiments, conductive composition 114 may have a reinforcement member113 embedded in the hydrogel, which may be a woven or a non-woven meshor any other material, such as a scrim, suitable for forming areinforcement member. Electrode 100 may also possess a conductive layer124, which may, in embodiments, be a suitable material such as aconductive carbon film of suitable thickness, in embodiments about 2mil. In some embodiments, a flood coat or a partial coating of silverink 115 (which can be silver and/or silver chloride) may be betweenconductive layer 124 and conductive composition 114, applied as acoating on at least a portion of a surface of conductive layer 124. Inembodiments, the electrode may include silver (Ag) orsilver/silver-chloride (Ag/AgCl) disposed on at least a portion of thefirst and/or second sides of the conductive layer.

Electrode 100 may also possess a standard stainless steel, tin/copper,or nickel plated pig tail lead wire 160 of a suitable length, inembodiments from about 5 to about 15 inches long, in other embodimentsabout 9 inches long. Lead wire 160 may possess an insulation jacketwhich, in turn, may be bound to conductive layer 124 using an adhesive170. Electrode 100 may also possess a reinforcement film 150 having amedical grade pressure sensitive adhesive (PSA) thereon overlying leadwire 160 and affixing reinforcement film 150 to both conductive layer124 and cover material 180. Finally, cover material 180 may possess pulltab 190 notched out of cover material 180 on the end of electrode 100opposite the end into which the lead wire 160 enters the electrode.

Medical electrodes may be packaged for use in any suitable materialswithin the purview of those skilled in the art. For example, electrodesmay be packaged in materials such as polyethylene or other plasticfilms, foil barrier packaging, combinations thereof, and the like.

INDUSTRIAL APPLICABILITY

The conductive compositions of the present disclosure may be useful withmedical electrodes that can be used with medical equipment for a varietyapplications, such as: electrocardiogram monitoring (ECG) electrodes(tab and snap style) for monitoring heart activity and for diagnosingheart abnormalities; electroencephalogram (EEG) electrodes;transcutaneous electrical nerve stimulation (TENS) electrodes used forpain management; neuromuscular stimulation (NMS) used for treatingconditions such as scoliosis; muscle stimulation electrodes; woundtreatment electrodes (accelerated healing of skin wounds or brokenbones); defibrillation electrodes to dispense electrical energy to achest cavity of a mammalian patient to defibrillate heart beats of thepatient; iontophoresis; and dispersive electrodes to receive electricalenergy dispensed into an incision made during electrosurgery. Otherapplications of the conductive compositions of the invention include,for example, electro-surgical dispersive pads; drug delivery (passive oriontophoretic); pre-surgical limb or area markers, tapes (anchoringchest tubes, NG tubes, IVs, cannulae, etc); and sterile seals at needleor cannula entry points. The medical equipment used in theseapplications is within the purview of those skilled in the art. Thepresence of one or more heat or cool sensation enhancing components mayaid in the dilation or contraction of pores in the skin, and may enhancesignal pickup and/or control skin permeability.

It will be appreciated that various of the above-disclosed and otherfeatures and functions, or alternatives thereof, may be desirablycombined into many other different systems or applications. Also thatvarious presently unforeseen or unanticipated alternatives,modifications, variations or improvements therein may be subsequentlymade by those skilled in the art which are also intended to beencompassed by the following claims. Unless specifically recited in aclaim, steps or components of claims should not be implied or importedfrom the specification or any other claims as to any particular order,number, position, size, or material.

What is claimed is:
 1. A conductive composition comprising: a conductivehydrogel formed of a crosslinked polymer; at least one component capableof providing a patient with a sensation selected from the groupconsisting of warming and cooling upon contact with tissue of a patient;and an additive selected from the group consisting of aloe, centellaasiatica, echinacea, ginkgo biloba, ginseng, hyssop, and combinationsthereof.
 2. The conductive composition of claim 1, wherein theconductive hydrogel comprises a component selected from the groupconsisting of gelatin, polysaccharides, crosslinked acrylamide polymers,hydroxyethylmethacrylate polymers, crosslinked polyhydroxyethylacrylate,polymerized, crosslinked 2-acrylamido-2-methylpropane sulfonic acidpolymers, crosslinked polyvinylpyrrolidone, polyacrylic acid, copolymersof the foregoing, one or more salts thereof, and combinations thereof.3. The conductive composition of claim 1, wherein the conductivehydrogel comprises a copolymer comprising a first monomer comprising amixture of acrylic acid and a salt thereof, present in an amount of fromabout 8 weight % to about 85 weight % of the hydrogel, and a secondmonomer of the formula CH₂═CHC(O)XR, in which X is O or NH and R is anunsubstituted or substituted alkyl group of from about 1 to about 5carbon atoms present in an amount of from about 15 weight % to about 92weight % of the conductive hydrogel.
 4. The conductive composition ofclaim 1, wherein the conductive hydrogel comprises a homogeneous mixtureof water and the crosslinked polymer present at a concentration of fromabout 7 percent by weight to about 35 percent by weight of saidhomogeneous mixture, said crosslinked polymer having a weight averagemolecular weight of from about 200 kD to about 5,000 kD.
 5. Theconductive composition of claim 4, wherein said crosslinked polymercomprises poly(ethylene oxide).
 6. The conductive composition of claim4, wherein said crosslinked polymer comprises poly(vinyl pyrrolidone)and a viscosity-enhancing hydrophilic polymer having a weight averagemolecular weight in excess of about 100 kD present at a concentration offrom about 1 to about 2 percent by weight of said homogeneous mixture,the viscosity-enhancing hydrophilic polymer selected from the groupconsisting of polyacrylamide, poly(vinyl alcohol), polacrylate,poly(ethylene oxide), poly(ethylene imine), carboxymethylcellulose,methylcellulose, cellulose derivatives, polyacrylamide sulfonic acid,polyacrylonitrile, agar, dextran, dextrin, carageenan, xanthan, guar,derivatives, mixtures, blends, and copolymers thereof.
 7. The conductivecomposition of claim 1, wherein the at least one component capable ofproviding a patient with a sensation selected from the group consistingof warming and cooling comprises a warming component selected from thegroup consisting of capsaicin, nonivamide, cinnamaldehyde, andcombinations thereof, present in an amount of from about 0.01% by weightto about 1% by weight of the conductive hydrogel.
 8. The conductivecomposition of claim 1, wherein the at least one component capable ofproviding a patient with a sensation selected from the group consistingof warming and cooling comprises a cooling component selected from thegroup consisting of menthol, camphor, eucalyptol, icilin, methyllactate, N-ethyl-p-menthane-3-carboxamide, and combinations thereof,present in an amount of from about 0.01% by weight to about 20% byweight of the conductive hydrogel.
 9. The conductive composition ofclaim 1, wherein the conductive hydrogel further comprises anelectrolyte present in an amount of from about 0.1% by weight to about15% by weight of the conductive hydrogel.
 10. The conductive compositionof claim 1, wherein the conductive hydrogel further comprises aneutralizer selected from the group consisting of ammonium hydroxide,sodium hydroxide, potassium hydroxide, lithium hydroxide, andcombinations thereof.
 11. The conductive composition of claim 1, whereinthe conductive hydrogel further comprises a cross linking agent selectedfrom the group consisting of N—N′-methylene bis-acrylamide, diethyleneglycol diacrylate, diethylene glycol dimethacrylate, trimethylolpropanetriacrylate, trimethylolpropane trimethacrylate, ethoxylatedtrimethylolpropane triacrylate, ethoxylated trimethylolpropanetrimethacrylate, pentaerythritol triacrylate, pentaerythritoltrimethacrylate, pentaerythritol tetraacrylate, pentaerythritoltetramethacrylate, polyethylene glycol diacrylate, polyethylene glycoldimethacrylate, and combinations thereof.
 12. The conductive compositionof claim 1, wherein the conductive hydrogel further comprises apolymerization initiator selected from the group consisting of2,2-azobisisobutyronitrile, 1-hydroxycyclohexylphenyl ketone,2-hydroxy-2-methyl-1-phenylpropan-1-one,2-hydroxy-1-[4-(2-hydroxyethoxy)phenyl]-2-methyl-1-propan-1-one,2,2-dimethoxy-2-phenylacetophenone, benzophenone, and combinationsthereof.
 13. A conductive composition comprising: a conductive hydrogelformed of a crosslinked polymer; at least one component capable ofproviding a patient with a sensation selected from the group consistingof warming and cooling upon contact with tissue of a patient; and anadditive selected from the group consisting of centella asiatica,echinacea, ginkgo biloba, ginseng, hyssop, and combinations thereof,wherein the warming component is selected from the group consisting ofnonivamide, cinnamaldehyde, and combinations thereof, and wherein thecooling component is selected from the group consisting of icilin,methyl lactate, and combinations thereof.
 14. The conductive compositionof claim 13, wherein the conductive hydrogel comprises a componentselected from the group consisting of gelatin, polysaccharides,crosslinked acrylamide polymers, hydroxyethylmethacrylate polymers,crosslinked polyhydroxyethylacrylate, polymerized, crosslinked2-acrylamido-2-methylpropane sulfonic acid polymers, crosslinkedpolyvinylpyrrolidone, polyacrylic acid, copolymers of the foregoing, oneor more salts thereof, and combinations thereof.
 15. The conductivecomposition of claim 13, wherein the conductive hydrogel comprises acopolymer comprising a first monomer comprising a mixture of acrylicacid and a salt thereof, present in an amount of from about 8 weight %to about 85 weight % of the hydrogel, and a second monomer of theformula CH₂═CHC(O)XR, in which X is O or NH and R is an unsubstituted orsubstituted alkyl group of from about 1 to about 5 carbon atoms presentin an amount of from about 15 weight % to about 92 weight % of theconductive hydrogel.
 16. The conductive composition of claim 13, whereinthe conductive hydrogel comprises a homogeneous mixture of water and thecrosslinked polymer present at a concentration of from about 7 percentby weight to about 35 percent by weight of said homogeneous mixture,said crosslinked polymer having a weight average molecular weight offrom about 200 kD to about 5,000 kD.
 17. The conductive composition ofclaim 16, wherein said crosslinked polymer comprises poly(ethyleneoxide).
 18. The conductive composition of claim 16, wherein saidcrosslinked polymer comprises poly(vinyl pyrrolidone) and aviscosity-enhancing hydrophilic polymer having a weight averagemolecular weight in excess of about 100 kD present at a concentration offrom about 1 to about 2 percent by weight of said homogeneous mixture,the viscosity-enhancing hydrophilic polymer selected from the groupconsisting of polyacrylamide, poly(vinyl alcohol), polacrylate,poly(ethylene oxide), poly(ethylene imine), carboxymethylcellulose,methylcellulose, cellulose derivatives, polyacrylamide sulfonic acid,polyacrylonitrile, agar, dextran, dextrin, carageenan, xanthan, guar,derivatives, mixtures, blends, and copolymers thereof.
 19. Theconductive composition of claim 13, wherein the warming component ispresent in an amount of from about 0.01% by weight to about 1% by weightof the conductive hydrogel.
 20. The conductive composition of claim 13,wherein the cooling component is present in an amount of from about0.01% by weight to about 20% by weight of the conductive hydrogel. 21.The conductive composition of claim 13, wherein the conductive hydrogelfurther comprises an electrolyte present in an amount of from about 0.1%by weight to about 15% by weight of the conductive hydrogel.
 22. Theconductive composition of claim 13, wherein the conductive hydrogelfurther comprises a neutralizer selected from the group consisting ofammonium hydroxide, sodium hydroxide, potassium hydroxide, lithiumhydroxide, and combinations thereof.
 23. The conductive composition ofclaim 13, wherein the conductive hydrogel further comprises a crosslinking agent selected from the group consisting of N—N′-methylenebis-acrylamide, diethylene glycol diacrylate, diethylene glycoldimethacrylate, trimethylolpropane triacrylate, trimethylolpropanetrimethacrylate, ethoxylated trimethylolpropane triacrylate, ethoxylatedtrimethylolpropane trimethacrylate, pentaerythritol triacrylate,pentaerythritol trimethacrylate, pentaerythritol tetraacrylate,pentaerythritol tetramethacrylate, polyethylene glycol diacrylate,polyethylene glycol dimethacrylate, and combinations thereof.
 24. Theconductive composition of claim 13, wherein the conductive hydrogelfurther comprises a polymerization initiator selected from the groupconsisting of 2,2-azobisisobutyronitrile, 1-hydroxycyclohexylphenylketone, 2-hydroxy-2-methyl-1-phenylpropan-1-one,2-hydroxy-1-[4-(2-hydroxyethoxy)phenyl]-2-methyl-1-propan-1-one,2,2-dimethoxy-2-phenylacetophenone, benzophenone, and combinationsthereof.
 25. The conductive composition of claim 13, wherein theadditive is present in an amount from about 0.1% by weight to about 10%by weight of the conductive hydrogel.